As fuel cells converting chemical energy into electrical energy are effective and do not generate an environmental pollutant, the fuel cells have attracted attention as clean power supplies for portable information devices, household use, vehicles and the like, and the development of the fuel cells have been taking place.
In recent years, as portable electronic devices such as cellular phones, notebook personal computers, digital cameras and camcorders have become more sophisticated and multifunctional, the portable electronic devices tend to consume more power. As power supplies for the portable electronic devices, typically small primary batteries and secondary batteries are used.
Measures of characteristics of batteries include energy density and output density. The energy density is the amount of electrical energy allowed to be supplied per unit mass or unit volume of a battery. The output density is an output per unit mass or unit volume of a battery. Improvements in energy density and output density in batteries used in portable electronic devices are desired to support more functionality and multifunctionality.
For example, at present, lithium-ion secondary batteries widely used as power supplies for portable electronic devices have good characteristics such as large output density. Moreover, in the lithium-ion secondary batteries, the energy density is relatively large, and the volume energy density reaches 400 Wh/L or over. However, unless a major change in constituent materials occurs in the lithium-ion secondary batteries, a further improvement in energy density is not expected.
Therefore, to support portable electronic devices expected to become more multifunctional and to consume more power, fuel cells are expected as power supplies for next-generation portable electronic devices.
In the fuel cell, a fuel is supplied to an anode to be oxidized, and air or oxygen is supplied to a cathode to be reduced, and in the whole fuel cell, a reaction in which the fuel is oxidized by oxygen occurs. As a result, chemical energy of the fuel is efficiently converted into electrical energy, and the electrical energy is extracted. Therefore, when the fuel is continuously supplied, unless the fuel cell is broken, the fuel cell is allowed to continue to be used as a power supply without being charged.
Various kinds of fuel cells have been proposed or prototyped, and some of the fuel cells have been put in practical use. These fuel cells are classified according to used electrolytes into alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid electrolyte fuel cells (SOFCs), polymer electrolyte fuel cells (PEFCs) and the like. Specifically, the PEFCs have such advantages that the used electrolyte is a solid, and the PEFCs are allowed to operate at a lower temperature than other types of fuel cells, for example, approximately 30° C. to 130° C., so the PEFCs are suitable as power supplies for portable electronic devices.
As fuels of the fuel cells, various combustible materials such as hydrogen and methanol are allowed to be used. However, as a cylinder for storage or the like is necessary for gas fuels such as hydrogen, the gal fuels are not suitable for downsizing of the fuel cells. On the other hand, liquid fuels such as methanol have such an advantage that the liquid fuels are easily stored. In particular, a direct methanol fuel cell (DMFC) in which methanol is directly supplied to an anode without being reformed to react has such advantages that a reformer for extracting hydrogen from the fuel is not necessary, the configuration of the DMFC is simple and the DMFC is easily downsized. Conventionally, the DMFC has been studied as one kind of PEFC, and the fuel cell of this kind is most likely to be used as a power supply for portable electronic device.
There are various types of fuel cells according to kinds of used electrolytes. Specifically, fuel cells using an organic material such as methanol or hydrogen as a fuel have attracted attention, and important constituent materials determining output performance of a fuel cell are an electrolyte material and a catalyst material, and a membrane-electrode assembly (MEA) configured by sandwiching an electrolyte membrane between catalyst films is an important constituent element.
As the electrolyte material, a large number of kinds of materials have been studied. For example, an electrolyte made of a perfluorosulfonic acid-based resin is a representative example. Moreover, as the catalyst material, a large number of kinds of materials have been studied. A PtRu catalyst is a representative example. In addition to the PtRu catalyst, to achieve a catalyst with high activity, a binary catalyst PtM including Au, Mo, W or the like as M has been studied.
For example, in the case where a bimetallic catalyst using Pt and Ru is used in a fuel electrode of the DMFC, by a deprotonation reaction represented by a formula (1), methanol is oxidized to produce CO, and CO is absorbed by Pt to produce Pt—CO. By a reaction represented by a formula (2), water is oxidized to produce OH, and OH is absorbed by Ru to produce Ru—OH. Finally, by a reaction represented by a formula (3), CO absorbed by Ru—OH is oxidized and removed as CO2 to generate an electron. Ru functions as a co-catalyst.Pt+CH3OH→Pt—CO+4H++4e−  (1)Ru+H2O→Ru—OH+H++e−  (2)Pt—CO+Ru—OH→Pt+Ru+CO2+H++e−  (3)
A principle of allowing methanol to be oxidized by the reactions represented by the formulas (1), (2) and (3) is widely known as bi-functional mechanism in which CO absorbed by Pt and a hydroxyl group bonded to Ru adjacent to Pt react with each other to convert CO into CO2, thereby preventing CO poisoning of a catalyst.
Aside from this, it is considered that under a condition where an electronic influence of Ru adjacent to Pt affects Pt, Pt—CO may be oxidized by H2O (water) by a reaction represented by a formula (4) after the reaction represented by the formula (1).Pt—CO+H2O→Pt+CO2+2H+2e−  (4)
The structural compositions of catalysts has been studied actively. A large number of catalysts including Pt and Ru with a core-shell structure which are applied to fuel cells have been reported (for example, refer to PTL1 to PTL4 which will be described later).
First, in PTL1 entitled “composite catalyst for fuel cell, method of manufacturing composite catalyst for fuel cell, method of manufacturing electrode catalyst layer for fuel cell and fuel cell” which will be described later, the following descriptions are given.
A composite catalyst for fuel cell of the disclosure of PTL1 is characterized by attaching an ion-exchange polymer (B) onto the surfaces of a core-shell type catalyst metal microparticles (A) each having a catalyst metal shell including catalyst metal and a metal core including a kind of metal different from the catalyst metal.
In the catalyst metal microparticles (A), as the catalyst metal forming a shell is not specifically limited, as long as the catalyst metal has catalytic activity for an electrode reaction of a fuel cell, and as the catalyst metal, Pt, Pd, Ir, Rh, Au, Ru, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ag, W, Re, Os or the like is used. Among them, noble metal such as Pt, Pd, Ir, Rh or Au is suitably used, and Pt is specifically preferably used because Pt has high catalytic activity.
Moreover, the metal forming the core in the catalyst metal microparticles (A) is not specifically limited, as long as the metal is different from the catalyst metal forming the shell, and, for example, Pt, Pd, Ir, Rh, Au, Ru, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ag, W, Re or Os is allowed to be used. The core may be made of a plurality of kinds of metals.
In the catalyst metal microparticles (A), the molar ratio (metal/catalyst metal) of the metal forming the metal core and the catalyst metal forming the catalyst metal shell is not specifically limited, and depending on the following particle diameter of the metal core, the following thickness of the catalyst metal shell or the like, typically, the molar ratio is preferably within a range of approximately 1/4 to 1/0.2. The particle diameter of the metal core and the thickness of the catalyst metal shell are not specifically limited, and may be appropriately set in consideration of, in addition to the utilization rate of the catalyst metal, dispersivity, an electronic effect, a steric effect or the like of a composite catalyst. In terms of stability of the core-shell structure and the utilization rate of the catalyst metal, the particle diameter of the metal core is preferably within a range of 1 to 50 nm, and specifically preferably within a range of 1 to 10 nm. It is necessary for the thickness of the catalyst metal shell to be equivalent to one atom or over of the catalyst metal, and in terms of the utilization rate of the catalyst metal, the thickness of the catalyst metal shell is preferably within a range of 0.3 to 5 nm, and specifically preferably within a range of 0.3 to 1 nm.
According to the disclosure of PTL1, catalyst metal microparticles (A) with a particle diameter of 1 to 10 nm are obtainable, and more specifically, catalyst metal microparticles (A) with a particle diameter of 2 to 10 nm are obtainable.
A simultaneous reduction method is a method of performing a reduction process on a mixed solution in which an ion-exchange polymer (b), a metal precursor (a2) and a metal catalyst precursor (a1) are dissolved to reduce metal ions and catalyst metal ions, thereby depositing core-shell type catalyst metal microparticles configured by forming the catalyst metal shells on surfaces of metal cores.
The ratio (feeding amounts) of the catalyst metal precursor (a1) and the metal precursor (a2) is not specifically limited, and the ratio of the catalyst metal shell and the metal core in the core-shell catalyst metal microparticle (A) is controlled by the feeding amounts, so the ratio may be appropriately set. Typically, the precursors are preferably fed so that the ratio (metal element/catalyst metal element) of a metal element of the metal precursor (a2) to a catalyst metal element of the catalyst metal precursor (a1) is 1/10 to 1/0.01, specifically 1/4 to 1/0.2. These are descriptions in PTL1.
Moreover, in PTL2 entitled “liquid containing dispersion stabilized catalyst nanoparticles” which will be described later, the following descriptions are given.
Catalytic nanoparticles include (a) nanoparticles containing a metal element Ru with a particle diameter of 1 nm to 10 nm and (b) a layer of platinum covering a part or a whole of a surfaces of the nanoparticle.
The particle diameter of the metal nanoparticle depends on the size of a platinum-group transition metal layer (for example, a Pt layer) reduced and supported on a surface thereof, but the particle diameter is preferably within a range of 1 nm to 10 nm. A particle diameter of 1 nm or less is not preferable, because an effect of preventing poisoning is not allowed to be sufficiently exerted on a surface of the supported platinum-group transition metal layer (for example, a Pt layer). Moreover, a particle diameter of 10 nm or over is not preferable, because metal atoms not involved in preventing poisoning on the surface of the platinum-group transition metal layer (for example, the Pt layer) are increased to cause an increase in cost. Note that the particle diameter is allowed to be evaluated by a scanning electron microscope or X-ray diffraction measurement.
Moreover, the covering rate of the Pt layer supported by the surface of the metal nanoparticle is not specifically limited, as long as a necessary surface area of Pt for achieving desired catalytic activity is secured, but the covering rate is preferably 5% or over of the surface area of the metal nanoparticle. Further, the thickness of the Pt layer is not limited, as long as the thickness of the Pt layer is thin enough for the metal nanoparticle to exert an influence on an electronic state of a Pt atom present on the surface of the Pt layer, but the thickness of one Pt atomic layer is preferably within a range of 3 nm or less. A thickness of 3 nm or over is not preferable, because the metal nanoparticle does not exert an influence on the electronic state of the Pt atom present on the surface of the Pt layer, and an effect of preventing carbon monoxide poisoning of a Pt surface is not allowed to be obtained. These are descriptions of PTL2.
Moreover, catalysts based on a result of a first-principles calculation have been studied (for example, refer to NPTL1 to NPTL3 which will be described later).
In the DMFC, typically, methanol which is a fuel as a low-concentration or high-concentration solution is supplied to an anode, and as represented by the following formula (a), methanol is oxidized in a catalyst layer on the anode side to produce carbon dioxide.Anode: CH3OH+H2O→CO2+6H++6e−  (a)
Hydrogen ions generated in the formula (a) are moved to a cathode side through a proton-conducting polymer electrolyte membrane sandwiched between the catalyst layer on the anode side and a catalyst layer on the cathode side, and as represented by the following formula (b), the hydrogen ions react with oxygen in the catalyst layer on the cathode side to generate water.Cathode: 6H++(3/2)O2+6e−→3H2O  (b)
A reaction occurring in the whole DMFC is represented by the following reaction formula (c) formed by combining the formulas (a) and (b).Whole DMFC: CH3OH+(3/2)O2→CO2+2H2O  (c)
The volume energy density of the DMFC is expected to reach a few times as large as that of a lithium-ion secondary battery. However, one issue of the DMFC is a small output density. Therefore, there is apprehension that when a fuel cell is designed to generate power for driving a portable electronic device only by itself, the size of the fuel cell exceeds an acceptable size for the portable electronic device; therefore the DMFC is not allowed to be contained in the portable electronic device.
Therefore, it is desired to improve the output density of the DMFC, and as one of measures to improve the output density, it is desired to improve activity of an anode catalyst for methanol oxidation. Conventionally, as the anode catalyst, a catalyst formed by forming microparticles of an alloy catalyst including platinum (Pt) and ruthenium (Ru) and supporting the microparticles of the alloy catalyst on conductive carbon material powder is typically used.\
A catalyst microparticle containing platinum exhibits high catalytic activity for oxidation of hydrogen or methanol. However, a pure platinum microparticle is easily poisoned by carbon monoxide (CO), and carbon monoxide is strongly absorbed on an active site of the platinum microparticle; therefore, the catalytic activity of the platinum microparticle may be pronouncedly reduced. Therefore, in the case where carbon monoxide is included in a hydrogen gas which is a typical fuel of the PEFC or in the case where carbon monoxide is produced as an intermediate as in the case of the DMFC, the pure platinum microparticle is not allowed to be used as the anode catalyst of the PEFC.
On the other hand, it is known that a platinum/ruthenium catalyst in which platinum coexists with a metal element such as ruthenium has good properties as an anode catalyst resistant to carbon monoxide poisoning, and a platinum/ruthenium alloy catalyst or the like is used as the anode catalyst of the PEFC. The catalysis of a catalyst including platinum and ruthenium for methanol oxidation is described by the bi-functional mechanism (refer to Electroanalytical Chemistry and Interfacial Electrochemistry, 60 (1975), pp. 267-273).
In the bi-functional mechanism, methanol gradually loses hydrogen (H) as hydrogen ions (H+) by a plurality of reactions on platinum to be oxidized, thereby generating carbon monoxide as represented by the following formula (d).CH3OH→CO+4H++4e−  (d)
On the other hand, as represented by the following formula (e), absorbed water is decomposed on ruthenium to produce a hydroxyl radical (OH).H2O→OH+H++e−  (e)
As represented by the following formula (f), carbon monoxide produced on platinum is oxidized by the hydroxyl radical on adjacent ruthenium to produce carbon dioxide (CO2), thereby completing oxidation of methanol.CO+OH→CO2+H++e−  (f)
As described above, in the catalyst including platinum and ruthenium, carbon monoxide on platinum is oxidized to produce carbon dioxide, and carbon monoxide is smoothly removed from the active site of platinum; therefore, the catalyst including platinum and ruthenium is resistant to carbon monoxide poisoning of platinum, and has high activity for methanol oxidation. According to the bi-functional mechanism, it is expected that a catalyst having more points where platinum and ruthenium are adjacent to each other has higher activity for methanol oxidation, and, for example, a catalyst in which platinum and ruthenium are mixed at atomic level, that is, a platinum/ruthenium alloy catalyst is considered to have high activity.
Actually, it is reported that in platinum/ruthenium-based catalysts, a catalyst in which platinum and ruthenium are mixed well at atomic level, that is, a catalyst in which alloying is proceeded has high activity for methanol oxidation (refer to Materials Research Society Symposium Proceedings, Vol. 900E, 0900-009-12). Moreover, in commercially available platinum/ruthenium catalysts, a catalyst allowing the DMFC to obtain a relatively high output when the catalyst is used as an anode catalyst of the DMFC is a catalyst with a high level of alloying of platinum and ruthenium.
Note that an alloy catalyst including platinum and ruthenium or the like is formed into microparticles to be used. It is because as noble metal such as platinum is expensive and rare, it is desired to reduce the amount of used platinum as small as possible; therefore, to form a high-activity catalyst containing a small amount of platinum, the area of a surface exhibiting catalytic activity is increased. Moreover, typically, catalyst microparticles are supported on conductive carbon material powder or the like to be used. It is because when the catalyst is formed into microparticles, a reduction in the surface area of the catalyst caused by the agglomeration of microparticles is prevented.
However, a method of reducing carbon monoxide poisoning by alloying to increase activity for methanol oxidation has a limit, and the catalytic activity for methanol oxidation of commonly available platinum/ruthenium alloy catalyst microparticles at present is not sufficient. Therefore, in the case where such platinum/ruthenium alloy catalyst microparticles are used as the anode catalyst of the DMFC, it is difficult to achieve a sufficiently high output density necessary for portable electronic devices. Moreover, it is desirable to reduce the amount of expensive platinum in the formation of the catalyst; however, when the amount of platinum is reduced, the catalytic activity is reduced.
One impediment to an improvement in catalyst performance of the platinum/ruthenium alloy catalyst is an insufficient reduction in particle diameter of the catalyst microparticle or insufficient alloying. In a typically method of producing the platinum/ruthenium alloy catalyst, platinum microparticles are supported on a carbon carrier, and then ruthenium microparticles are supported, and after that, platinum microparticles and Ru microparticles are fused by heating to be alloyed. In this producing method, unless the platinum microparticles and the Ru microparticles are grown to a size to some extent or over, alloying is insufficient, and a reduction in particle diameter of the catalyst microparticle and alloying are conflicting needs, so it is difficult to solve this issue. Moreover, the smaller the amount of supported platinum microparticles or ruthenium microparticles on the carbon carrier is, the more sparsely the platinum microparticles and the Ru microparticles are distributed; therefore, it is difficult to fuse and alloy the platinum microparticles and the Ru microparticles.
Another impediment to the improvement in catalyst performance of the alloy catalyst is not allowing a platinum atom present inside the catalyst microparticle to be used. In atoms forming the catalyst microparticle, only an atom present near the surface of the catalyst microparticle involves catalysis. Therefore, in the platinum/ruthenium alloy catalyst microparticle, a platinum atom present inside the catalyst microparticle does not contribute to catalysis. The larger the particle diameter of the catalyst microparticle is, the more the ratio of platinum atoms which do not contribute to catalysis and are wasted is increased. Moreover, there is an issue that in the platinum/ruthenium alloy catalyst microparticle, ruthenium is eluted into an electrolyte during power generation of the DMFC, thereby gradually reducing catalytic activity. In the alloy catalyst, in order to perform catalysis, it is necessary for platinum and ruthenium to be exposed to the surface of the catalyst. In the alloy catalyst, it is difficult to solve an issue of durability that ruthenium is eluted during power generation.
In addition to the platinum/ruthenium alloy catalyst, anode catalysts for DMFC including various materials such as a catalyst formed by combining platinum and an element except for platinum and a catalyst not using platinum for cost reduction have been studied, but all of the catalysts have an issue of catalytic activity or durability. Therefore, to achieve a high output density necessary for portable electronic devices in the DMFC, a different approach is desired.
In PTL5 which will be described later, catalyst microparticles each including a ruthenium particle and a platinum layer covering a part of a surface of the ruthenium particle is proposed as an electrode catalyst for fuel cell. In the catalyst microparticles, an alloying process is not provided, so the particle diameter of the catalyst microparticle is not limited by the alloying process. Moreover, platinum atoms are not present inside the catalyst microparticles, and platinum atoms are present only on the surfaces of the microparticles, so the amount of wasted platinum is allowed to be reduced, and compared to alloy microparticles, platinum use efficiency is high.
In PTL5, as a method of producing the above-described catalyst microparticles, a producing method including a step of producing ruthenium colloid particles by adding a reducing agent to a ruthenium salt solution, a step of bubbling hydrogen in a dispersion liquid in which the ruthenium colloid particles are dispersed for allowing hydrogen to be adsorbed to the surfaces of the ruthenium colloid particles, and a step of adding a solution containing platinum salt to the dispersion liquid to form platinum layers on the surfaces of the ruthenium particles by reducing platinum ions by hydrogen on the surfaces of the ruthenium particles.
To reduce the particle diameter of the catalyst microparticle to achieve high catalyst performance, in a producing process, it is necessary to stably disperse ruthenium particles and prevent agglomeration or the like. Therefore, PTL5 describes that when the ruthenium particles are synthesized, an anti-agglomeration agent for preventing the agglomeration of the ruthenium particles such as polyvinylpyrrolidone is preferably added to the ruthenium salt solution.
However, by doing so, an active site of the catalyst is covered with the anti-agglomeration agent, so after catalyst microparticles are synthesized, for example, heat treatment for removing the anti-agglomeration agent such as heating the catalyst at approximately 300° C. in the presence of hydrogen is necessary. There is apprehension that in the heat treatment, the agglomeration of catalyst microparticles occurs to reduce the active surface area of the catalyst.
In PTL2 which will be described later, there are proposed a colloid solution in which catalyst nanoparticles each including a nanoparticle having a metal element such as ruthenium and a layer of platinum or the like covering a part or a whole of the surface of the nanoparticle are stably dispersed by a carboxylic compound such as citric acid, and a nanoparticle-containing catalyst in which the colloid solution and a carrier are mixed to support catalyst nanoparticles on the carrier.
Moreover, in PTL2, there is proposed a method of producing a colloid solution by preparing a metal salt-containing liquid containing a carboxylic compound such as citric acid as a dispersion stabilizer, and applying the liquid to colloid formation to produce a nanoparticle having a metal element such as ruthenium, and then adding a platinum-group metal salt-containing liquid to perform a reduction process, thereby forming a layer of platinum or the like covering a part or a whole of surface of nanoparticle.
A characteristic of the method of producing a colloid solution proposed in PTL2 is using a carboxylic compound such as citric acid instead of a polymer-based anti-agglomeration agent such as polyvinylpyrrolidone used in the producing method in PTL5. According to PTL2, the carboxylic compound has small absorption force to the surfaces of the catalyst nanoparticles; therefore, when the carboxylic compound is used as a catalyst, the carboxylic compound is removed from surfaces of the catalyst nanoparticles. Therefore, the heat treatment for removing the carboxylic compound is not necessary, and there is no possibility that the agglomeration of catalyst microparticles in the heat treatment occurs. Moreover, even if the heat treatment is performed, the heat treatment is allowed to be performed at a lower temperature than that in the case where the polymer-based anti-agglomeration agent is removed, so there is a less possibility that the agglomeration of the catalyst microparticles occurs to cause a reduction in the active surface area.
FIG. 28 is a schematic view of a catalyst illustrated in FIG. 1 in PTL2. In the catalyst, parts of the surface of one Ru nanoparticle 101 are covered with a plurality of island-shaped platinum layers 102, and a catalyst microparticle 103 formed in such a manner is supported on a carbon carrier 104. Moreover, a carboxylic compound 105 such as citric acid used for producing a colloid solution is adhered to the surfaces of the Ru nanoparticle 101 and the platinum layer 102.